Periodic testing of ultrasound scanners and transducers is important for maintaining image quality, and is a required element of ultrasound practice accreditation programs. A new federal law, which goes into effect in 2012 in the United States, requires echocardiography imaging practices to achieve and maintain accredited status in order to be reimbursed for imaging examinations of Medicare and Medicaid patients. This new legislation, as well as similar requirements imposed by private insurance providers, has increased the focus on accreditation, and performance evaluation of medical imaging equipment including ultrasound scanners.
As understood by a person of skill in the art, there are a variety of types of phantoms used to support the quality assurance and performance testing of ultrasound scanners. Recommended tests include an element or channel failure (EOCF) test, a distance measurement accuracy (DMA) test, and a maximum depth of penetration (DOP) test. In the EOCF test, images of a macroscopically uniform phantom are inspected for the presence of shadowing extending from the transducer surface. The potential clinical significance of any detected shadowing is assessed. In the distance measurement accuracy test, the accuracy of distance measurements made in the vertical and horizontal directions using a phantom containing multiple objects is determined. In the DOP test, an assessment of the maximum depth from which echoes can be detected is measured. For example, U.S. Pat. No. 5,670,719 to Madsen, et al., discloses a phantom containing background material mimicking the ultrasonic characteristics of human tissue and coplanar spherical target lesions ultrasonically contrasting with the background material. Digitized images can be formed from the ultrasound scan of slices in which the lesions are centered. A lesion signal to noise ratio, LSNR, is calculated at each sphere in the target lesion slice. This calculation employs (1) a pixel value average calculated over a sample area of the target image slice centered at a pixel location and of a size approximately that of the cross-sectional area of the target lesions, (2) an average pixel value calculated over an averaging area centered at the same pixel coordinate, but containing mostly background image data, and (3) a standard deviation of averaged pixel values calculated in the background material image plane. The proximal and distal depth range limits of detectability of an ultrasound scanner, for a given lesion diameter and contrast, can be determined based on the number of pixel locations in a depth range of the scanned plane having an absolute value LSNR greater than a threshold value.
For quality assurance and performance testing, most phantoms contain objects to be detected that are suspended in a material that closely mimics the ultrasonic propagation characteristics of human tissue. For example, a phantom may contain metal or plastic fibers, spheres, cylinders, etc, that may be oriented in a particular direction, such as perpendicular to the scanning plane, at known locations or at random, unknown locations. For example, U.S. Pat. No. 4,843,866 to Madsen, et al., discloses a phantom including a multiplicity of scattering particles spaced sufficiently close to each other that the scanner is incapable of resolving individual scattering particles and testing spheres having a testing sphere ultrasonic speed, specific gravity, attenuation coefficient, and backscatter coefficient, at least one of which is different from the corresponding suspension material ultrasonic speed, specific gravity, attenuation coefficient, and backscatter coefficient. The testing spheres are located within the phantom in a random array.
U.S. Pat. No. 6,238,343, to Madsen, et al., discloses an ultrasound phantom configured to provide multiple testing capabilities for quality assurance on ultrasound scanners. The phantom includes a section with an array of target spheres that allow the ability of the scanner to delineate the intersection of a plane of symmetry of the scan slice with diagnostic objects to be determined. Other sections allow low contrast resolution of large objects, spatial resolution regarding lateral and axial dimensions, maximum visualization depth, image gray level uniformity, and distance measurement accuracy to be determined. The phantom may be utilized to provide comparative tests of various scanners and to monitor the performance of a particular scanner over time to determine any changes in the performance of the scanner.
Ideally, the suspension materials are capable of mimicking soft human tissue with respect to at least three characteristics: speed of sound, ultrasonic attenuation, and ultrasonic backscattering. The attenuation coefficient should be approximately proportional to the ultrasonic frequency. In other words, the variation of the attenuation coefficient with respect to frequency, or the attenuation coefficient slope, should remain constant approximately constant over a range of ultrasonic frequencies. The attenuation coefficient should extrapolate to zero as the frequency reduces to zero. These characteristics of human tissue should be maintained at all frequencies in the typical range of ultrasonic scanners, from 1 to 15 megahertz (MHz). Moreover, the variation of these characteristics within the range of room temperature should be small and the materials should be stable in time and invulnerable to reasonable environmental fluctuations.
A tissue mimicking material satisfying the above characteristics was disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
U.S. Pat. No. 5,625,137 to Madsen, et al. discloses a tissue mimicking material with intrinsic very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material.
The tissue mimicking material is enclosed in a container which includes an ultrasound transmitting window. For example, U.S. Pat. No. 6,190,915, to Madsen, et al., discloses an ultrasound transmitting window cover that seals and protects a water-based tissue mimicking material within the phantom container. The window cover includes a multi-layer film formed of at least a layer of metal adhered to a layer of plastic. The metal layer is essentially impervious to moisture and air molecules, preventing both desiccation of the water based material within the phantom and oxidation or contamination of the tissue mimicking material.
Ultrasound scanners for medical imaging are available from several manufacturers, in various models, with corresponding variations in the performance of the scanners. Additionally, each model of scanner may include one or more transducers of various shapes and sizes and having different types of sensors. For example, the transducers may include a single sensor or multiple sensor elements forming a linear or two-dimensional array, including a phased array. The types of arrays may include a linear array or an arc-shaped array (convex array). The radius of curvature (ROC) of the convex array can vary from 0.5 mm to 7 cm. The different types of arrays may operate at frequencies in the range of from 1 to 15 MHz or more.